T-cell exhaustion is a state of T-cell dysfunction that arises during many chronic infections and cancer. It is defined by poor T-cell effector function, sustained expression of inhibitory receptors and a transcriptional state distinct from that of functional effector or memory T-cells. Exhaustion prevents optimal control of infection and tumors. (E John Wherry, Nature Immunology 12, 492-499 (2011)).
T-cell exhaustion is characterized by the stepwise and progressive loss of T-cell functions. Exhaustion is well-defined during chronic lymphocytic choriomeningitis virus (LCMV) infection and commonly develops under conditions of antigen-persistence, which occur following many chronic infections including hepatitis B virus, hepatitis C virus and human immunodeficiency virus infections, as well as during tumor metastasis. Exhaustion is not a uniformly disabled setting as a gradation of phenotypic and functional defects can manifest, and these cells are distinct from prototypic effector, memory and also anergic T cells. Exhausted T cells most commonly emerge during high-grade chronic infections, and the levels and duration of antigenic stimulation are critical determinants of the process. (Yi et al., Immunology April 2010; 129(4):474-481).
Circulating human tumor-specific CD8+ T cells may be cytotoxic and produce cytokines in vivo, indicating that self- and tumor-specific human CD8+ T cells can reach functional competence after potent immunotherapy such as vaccination with peptide, incomplete Freund's adjuvant (IFA), and CpG or after adoptive transfer. In contrast to peripheral blood, T-cells infiltrating tumor sites are often functionally deficient, with abnormally low cytokine production and upregulation of the inhibitory receptors PD-1, CTLA-4, TIM-3 and LAG-3. Functional deficiency is reversible, since T-cells isolated from melanoma tissue can restore IFN-γ production after short-term in vitro culture. However, it remains to be determined whether this functional impairment involves further molecular pathways, possibly resembling T-cell exhaustion or anergy as defined in animal models. (Baitsch et al., J Clin Invest. 2011; 121(6):2350-2360).
Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), also called CD152, is a type I transmembrane protein encoded in humans by the CTLA4 gene. The molecular properties and biological functions of CTLA-4 described herein are reviewed in McCoy and Le Gros Immunology and Cell Biology (1999) 77: 1-10 and Grosso and Kunkel, Cancer Immunity (2013) 13: 5.
Binding of the positive costimulatory receptor CD28 to its ligands CD80 and CD86 on antigen presenting cells (APCs) leads to activation of T cells, resulting in T cell proliferation and production of interleukin-2 (IL-2). CTLA-4 is expressed at the cell surface of activated CD4+ and CD8+ T cells, and is an important negative regulator of T cells function. CTLA-4 has a structure similar to CD28, and also binds to both CD80 and CD86 on APCs, but with greater avidity and affinity (Collins et al., Immunity (2002) 17: 201-210).
CTLA-4 has been shown to negatively regulate immune activation through both intrinsic and extrinsic mechanisms, summarised in Table 1 of Grosso and Kunkel, Cancer Immunity (2013) 13: 5. Briefly, (i) reverse signalling through CD80 and CD86 on APCs results in suppression of T cell responses and/or promotes conversion of naïve T cells to Tregs, (ii) signaling through CTLA-3 stimulates production of regulatory cytokines such as TGF-β, resulting in inhibition of antigen presentation by APCs and inhibition of T cell function, (iii) binding of CTLA-4 to CD80/CD86 reduces availability of these ligands for binding by CD28, resulting in reduced activation of T cells by APCs, (iv) binding of CTLA-4 to CD80/CD86 causes their transendocytosis, reducing the ability for APCs to activate T cells, (v) CTLA-4 recruits inhibitory proteins such as PP2A and PTPN11 to the T cell synapse, inhibiting signalling through CD28 and TCR, (vi) CTLA-4 acts as a high affinity competitor occupying CD80/86 and thereby preventing binding by CD28, (vii) a soluble splice variant of CTLA-4 may be capable of inhibiting T cell activation, and (viii) CTLA-4 inhibits the T cell stop signal, which is important for activation of T cells by APCs.
Inhibition of negative regulation by CTLA-4 has been shown to promote stimulation of adaptive immune response and T cell activation. CTLA-4-blocking antibodies have been shown to be efficacious in mouse models of cancer, and anti-CTLA-4 antibodies such as ipilimumab (Yervoy, MDX-010, 10D1; described in WO2001014424 A1) and tremelimumab (ticilimumab; CP-675,206) are being investigated as strategies to promote anti-tumor immunity in cancer. Blockade of CTLA-4 is also a promising therapeutic strategy for disorders associated with T cell exhaustion such as chronic viral infection.
Ipilimumab has been demonstrated not to be capable of binding to murine CTLA-4 (WO 2001/1014424 A1, Table 5, page 81), and tremelimumab has likewise been shown not to bind to murine CTLA-4 (Hanson et al. Proc Amer Assoc Cancer Res (2004) 64: 877). Hanson et al. also discloses that tremelimumab displays binding to human CD28.